ELECTRON-MICROSCOPY INVESTIGATION OF SUPERCONDUCTING LA2CU(O, F)4+Y OXYFLUORIDE

The fluorination of La2CuO4 can lead to different oxyfluoride compound s depending on the TF2 temperature of the fluorine gas treatment. When 150-degrees-C T(F2) less-than-or-equal-to 200-degrees-c less-than-or- equal-to 200-degrees-C a superconducting material is obtained. Previou s neutron diffraction experiments as well as the EXAFS measurements at the La L(III) edge indicate that extra anions lie in an interstitial site between the two (LaO) layers. Electron diffraction patterns clear ly show the existence of an incommensurate modulation due to the prese nce of shear planes. A second phase is also pointed out which can be o btained as a major component when the fluorination temperature is rais ed to 230-degrees-C. This phase which is not a superconductor crystall izes with the monoclinic symmetry.